The present invention relates to thermal printing systems and, more particularly, to a method and apparatus for controlling the heaters on the thermal print head.
A thermal printing system utilizes a thermal print head which includes a substrate onto which a line of resistive heat-generating elements or heaters is deposited. The resistive heaters are uniformly deposited in a single line and very closely together, typically with a resolution of 200 or 300 heaters per inch. An electric current is selectively and controllably applied to each of the heaters in accordance with the information to be thermally transferred to a corresponding pixel on a piece of paper or other medium adjacent to the thermal print head. Usually, the printing is accomplished by thermal transfer between a ribbon and the piece of paper. However, printing can also be accomplished using thermally sensitive paper. Printing on a medium with a thermal print head can be carried out by a process which generates the desired pattern on the paper one line at a time by selectively energizing the heaters as the paper is transferred past the thermal print head. Individual heaters are energized to levels corresponding to the desired gray levels of the pixels printed by the particular heaters. This is frequently accomplished by energizing the individual heaters repeatedly, with the number of times corresponding to the desired gray levels. This technique has the advantage of spreading out the heating of the elements in time thereby allowing for accurate dye transfer.
Another technique for energizing the individual heating element is described in U.S. Pat. No. 5,636,331 entitled xe2x80x9cPATTERNED INTENSITIES PRINTERxe2x80x9d which issued on Jun. 3, 1997 to Klinefelter et al. which is assigned to the same Assignee as the present application and is incorporated herein by reference. The technique described in Klinefelter et al. is advantageous because it requires fewer strobes of the heating element and is therefore faster than the simple pulsing technique mentioned above.
A printer for printing an image onto a substrate includes a thermal print head having a plurality of electrical resistors. A supply of coloring material adjacent the print head is provided for deposition on the substrate and a print head controller includes a pulse train output to at least one of the electrical resistors. The pulse train comprises a plurality of pulses. At least one pulse has a variable width related to a binary value and another pulse has fixed width.